An Accurate Model to Calculate CO2 Solubility in Pure Water and in Seawater at Hydrate–Liquid Water Two-Phase Equilibrium

Mengyao Di,Rui Sun,Wanjun Lu,Lantao Geng

doi:10.3390/min11040393

Abstract

Understanding of CO2 hydrate–liquid water two-phase equilibrium is very important for CO2 storage in deep sea and in submarine sediments. This study proposed an accurate thermodynamic model to calculate CO2 solubility in pure water and in seawater at hydrate–liquid water equilibrium (HLWE). The van der Waals–Platteeuw model coupling with angle-dependent ab initio intermolecular potentials was used to calculate the chemical potential of hydrate phase. Two methods were used to describe the aqueous phase. One is using the Pitzer model to calculate the activity of water and using the Poynting correction to calculate the fugacity of CO2 dissolved in water. Another is using the Lennard–Jones-referenced Statistical Associating Fluid Theory (SAFT-LJ) equation of state (EOS) to calculate the activity of water and the fugacity of dissolved CO2. There are no parameters evaluated from experimental data of HLWE in this model. Comparison with experimental data indicates that this model can calculate CO2 solubility in pure water and in seawater at HLWE with high accuracy. This model predicts that CO2 solubility at HLWE increases with the increasing temperature, which agrees well with available experimental data. In regards to the pressure and salinity dependences of CO2 solubility at HLWE, there are some discrepancies among experimental data. This model predicts that CO2 solubility at HLWE decreases with the increasing pressure and salinity, which is consistent with most of experimental data sets. Compared to previous models, this model covers a wider range of pressure (up to 1000 bar) and is generally more accurate in CO2 solubility in aqueous solutions and in composition of hydrate phase. A computer program for the calculation of CO2 solubility in pure water and in seawater at hydrate–liquid water equilibrium can be obtained from the corresponding author via email.

Highlights

There has been a growing concern on the potential impact of rising greenhouse gas levels in the atmosphere
One is using the Pitzer model [42] to calculate the activity of water and using the Poynting correction to calculate the fugacity of CO2 dissolved in water, as we did for hydrate–liquid water equilibrium (HLWE) of CH4 hydrate [97]
Another is using the Statistical Association Fluid Theory (SAFT)-LJ equation of state (EOS) [55] to calculate the activity of water and the fugacity of dissolved CO2

Summary

Introduction

There has been a growing concern on the potential impact of rising greenhouse gas levels in the atmosphere. Proposed sites for CO2 geological storage range from mined salt caverns, saline aquifers, depleted hydrocarbon reservoirs, unmineable coal seams, and so on [1]. Storages of CO2 in depleted oil/gas reservoirs and in saline aquifers are more technically feasible [2,3]. Injecting CO2 into the deep ocean to form CO2 hydrate is attractive since the CO2 storage capacity of deep oceans is huge [5]. This method is more controversial than other CO2 storage methods due to the risk of leakage, local acidification of seawater, the correspondingly negative impact on the marine environment, and so on [4,6,7]. In a hybrid method between ocean and geological storage, CO2 would be injected into submarine sediments below the deep ocean floor to avoid the potential leakage of CO2 and harm to marine ecosystems [9]

Objectives

Discussion

Conclusion